Also, SimpleSample has some very simple examples covering development of a filter that does virtually nothing through to one that draws a variable sized square, in the middle of the screen, in all ColorSpaces.

Once you've got the basics down on AVS development (Ben's text is quite good), the [SDK] for VirtualDub is also a good read. Good news is you won't have to worry about writing [function pointers] and [raw Win32]; meanwhile, Avery knows his stuff when it comes to video & CPU optimization techniques, so you best pay attention.